1. Field of the Invention
The present invention relates to a mobile telecommunication system which is adapted to secure a communication link for a mobile unit moving in a wide area by utilizing wireless communications, and more particularly to a mobile telecommunication system which is adapted to perform communication by utilizing a CDMA/TDD system adopting the direct sequence spread spectrum (DS-SS) system.
2. Description of the Related Art
A system in which communication is established among a plurality of stations in the same frequency band is called a multiple access system. CDMA (Code Division Multiple Access) is a technology in which the multiple access is performed on the basis of spread spectrum communication, in which an information signal is transmitted with the spectrum thereof spread into a sufficiently wider band than the band width of the information signal of the unit. A DS-SS system is a system wherein in the spreading, the information signal is directly multiplied by a spreading code. TDD (Time Division Duplex) means a system wherein the transmission/reception of the signal is performed in the same band and is also called a ping pong system, i.e., it means a system wherein the communication is performed with the same radio frequency subjected to time division in the transmission/reception. As for the advantages of the TDD system, as shown in a paper entitled "Micro/Pico Cellular Telecommunication and Network Architecture" (N. Nakajima: the 6-th Karuizawa Workshop on Circuits and Systems (Apr. 19 to 20, 1993) pp. 121 to 126), it is well known that since transmission diversity can be applied to the base station, space diversity becomes unnecessary in the mobile radio telephone, and as a result, miniaturization can be realized.
The received radio wave via the mobile propagation path is subjected to fluctuation called fading. This becomes a degradation factor of the transmission system. In order to realize high quality communication, as for the technology for reducing the influence of the above-mentioned fading, diversity reception utilizing two or more received radio waves is well known. Space diversity is one of the above-mentioned diversity technologies in which, by using two or more sufficiently spaced apart receiving antennas, a plurality of fading received radio waves are obtained which change independently of one another.
On the other hand, transmission diversity means a technology wherein from the received radio waves received via space diversity, the conditions of the path from the mobile station to the respective antennas of the base station are estimated, and transmission is performed in turn from the antenna having the better transmission path or link state. In the case of the TDD system, since the system of interest is a system wherein the transmission/reception is performed in the same band, the frequency correlation of the fading fluctuation of the transmitted radio wave is the same as that of the received radio wave. Therefore, if the interval of time required for the transmitted radio wave and the received radio wave to be switched to each other is sufficiently short, since the mutual time correlation of the fading fluctuation is high, by the application of the above-mentioned transmission diversity, a reduction of the influence by the fading fluctuation of the transmitted radio wave can be relatively readily promoted. As a result, for the individual channels, high transmission link quality can be realized.
Heretofore, the application of transmission diversity in a base station adopting the TDD system has been considered in a TDMA/TDD system. TDMA (Time Division Multiple Access) is a system wherein the radio frequency is subjected to time division, and a specific time slot is assigned to a user, and in the time slot thus assigned, the communication is performed.
The TDMA/TDD system is employed in a PHS (Personal Handy phone System) as the Japanese next generation digital codeless telephone system as well as in a DECT system which is in development in Europe. FIG. 1 shows an example in which in the PHS system, a base station BS is provided with two transmission/reception antennas A and B, and communication is established between the base station BS and four mobile terminals PS1 to PS4. The PHS system has a frame structure as shown in FIG. 2 for example. In this connection, with 5 msec (transmission 2.5 msec/reception 2.5 msec) as one frame, four channels are subjected to time division multiplexing. Each subframe, in the figure, for accommodating a signal of the associated channel is called a time slot, and the shadowed portions represent gard time intervals which are provided in order to prevent the transmitted signal and the received signal from colliding with each other due to a lag between the transmission timing and the receiving timing.
In addition, FIG. 3 is a view showing an example of a situation in which the base station BS, having the two antennas A and B as shown in FIG. 1, switches the transmission antenna every channel in accordance with the levels of the received signals. The transmission/reception frames and the time slots are shown. In FIG. 3, reference symbol TX represents transmission and reference symbol RX represents reception. Then, it is assumed that the four mobile terminals PS1 to PS4 perform their respective communications using the channels 1 to 4. In the case where at time t0, the levels of the received signals in the individual channels (the averages or the like of the reception levels between the time slots) have the relationship, as shown in the figure, with respect to the two antennas A and B (the relation of A&gt;B represents that the reception level of the antenna A is higher than that of the antenna B), when at the next time t1, the transmission is performed, the antenna having the higher reception level is selected to transmit the signal. In the figure, the reference symbol of the selected antenna is shown. At time t1, it is shown that each of the channels 1 and 3 selects the antenna A, and each of the channels 2 and 4 selects the antenna B. In the figure, a time slot for transmission is represented by a shadowed portion. In addition, a portion having no shadowing represents a time slot in which no transmission is performed. Similarly, at time t3, on the basis of the results (received power) at time t2, an antenna for transmission is selected every channel. In such a way, the above-mentioned transmission diversity can be realized.
As compared with such a TDMA system, the CDMA system is considered as a system wherein, when used in a cellular system, a higher frequency utilization efficiency than that in the TDMA system can be realized, and hence a larger number of users can be accommodated. Therefore, it is considered that in the future, the CDMA system will be applied to a large number of cellular systems. In addition, the TDD system is a system wherein the transmission/reception is performed in the same frequency band on the basis of time division, whereas as another communication system, there is well known an FDD (Frequency Division Duplex) system in which two frequency bands, which are sufficiently spaced apart from each other, are respectively assigned to transmission and reception. Heretofore, in the CDMA system, the CDMA/FDD system employing the FDD system has been mainly developed.
However, in the conventional communication apparatus adopting the CDMA/FDD system, there arise the following problems.
(1) Since the correlation between the fading of the forward link and that of the reverse link is small, the space diversity technique in the base station can be applied to only the reverse link.
(2) In the case where the received power is reduced due to frequency selective fading, the communication quality is degraded and hence it is difficult to perform high quality communication.
(3) In the case where the communication capacity has greatly increased, since the interference of the communication radio waves of other stations with the communication radio wave of the local station becomes large, the communication quality is degraded, and hence it is difficult to perform high quality communication.
(4) In the case where the communication capacity has greatly increased, the requirements on the quadrature modulator and the linear amplifier of the base station become severe.